The present invention relates to a flow controller for controlling comparatively low flow rates of a fluid passing through fluid paths such as pipelines, a method for testing the accuracy of the flow control provided by the flow controller to control the flow rate through the fluid path, and a flow control method.
Various types of semiconductor products or electronic components are manufactured, for example, by placing wafers in a chamber of a CVD (Chemical Vapor Deposition) apparatus and then supplying into this chamber a process gas that contains raw materials and chemically reactive substances necessary for deposition, thereby fabricating semiconductor integrated circuits on the surface of the wafers. The raw material gas used includes, for example, SiH4, WF6, and NH3. In addition, etching gases for use in etching after deposition include, for example, CH4 or Cl2.
Recently, there has been increasing demand for further reduction in size and higher functionality of semiconductor products and electronic components, which has led to semiconductor integrated circuits formed on wafers using finer design rules. Accordingly, those semiconductor integrated circuits would likely degrade in quality unless precise and high-speed control is performed on the flow rate of a process gas supplied to the semiconductor manufacturing apparatus such as a CVD apparatus. On the other hand, since semiconductor manufacturing lines involving manufacturing apparatuses such as a CVD apparatus require a significant capital investment, there has also been a strong demand for improvement in the rate of operation of the semiconductor manufacturing apparatuses.
To supply the process gas into such a semiconductor manufacturing apparatus at flow rates with high accuracy, it has been conventionally practiced to provide a mass flow controller in a pipeline (fluid path), through which the process gas passes, in order to regulate the flow rate of the process gas to a target value.
Conventionally employed mass flow controllers (hereinafter referred to as the “flow controller”) includes a flow rate sensor for measuring the flow rate through a fluid path and a flow control valve mechanism for regulating the flow rate through the fluid path. The flow controller also includes a controller for controlling the valve opening of this flow control valve mechanism so that the flow rate is regulated to a target flow rate value (hereinafter referred to as the “flow rate set value”) specified by an external system or the like. Furthermore, this controller computes the amount of difference (the amount of deviation) between the detected flow rate value received from the flow rate sensor and the flow rate set value. Then, to regulate the flow rate through the fluid path to the specified flow rate set value based on the amount of deviation, the controller delivers the amount of control (control signal) determined by PID operation to the flow control valve mechanism and hence regulates its valve opening, thereby providing control to adjust the flow rate to the flow rate set value.
The flow controller configured in this manner is required to perform flow control to make the flow rate through the fluid path coincide with the flow rate set value with high accuracy. This flow rate set value is specified, for example, using a flow rate setting signal, from an upper-class control system (external system) which collectively controls and monitors the operation of the semiconductor manufacturing line. However, with the various types of apparatus and the flow controller, including the semiconductor manufacturing apparatuses, constituting the semiconductor manufacturing line, there may occur changes in performance (hereinafter referred to as the “equipment aging”). The equipment aging refers to the phenomenon in which there occur slight changes in performance or characteristics of each member constituting the apparatus, due to adhesion of foreign substances. The equipment aging also refers to degradation in performance or characteristics of the members themselves over time from the start of operation of the manufacturing line.
More specifically, the equipment aging can be caused, for example, by adhesion of chemical products to the inside of a pipeline through which the process gas is supplied, by adhesion of chemical products to sensor pipes or bypass pipes constituting the flow rate sensor, or by degradation in performance of an actuator installed in the flow control valve mechanism. Once such equipment aging occurs, there would likely occur a slight difference in the valve opening with respect to the one initially available. This may happen even if the same valve drive control information (for example, valve drive voltage) as that initially employed at the time of introduction of the semiconductor manufacturing apparatus is delivered (applied) to the actuator included in the flow control valve mechanism of the flow controller. As a result, even if the flow controller provides control to adjust the flow rate through the fluid path to the flow rate set value, there will be found a deviation between the actually controlled flow rate and the flow rate set value.
Those problems concerned with such equipment aging have been addressed conventionally. To this end, the relevant semiconductor manufacturing apparatus has been stopped periodically or non-periodically to determine whether the flow controller installed in the fluid path can provide flow rate control as designed, i.e., to perform the operation of testing the accuracy of the flow control.
For example, the operation of testing the flow rate was carried out as follows.
A test tank having a known capacity was installed in the fluid path (pipeline) which was located upstream of the flow controller and through which a process gas was supplied. Subsequently, the process gas was allowed to flow stably through the fluid path at a predetermined flow rate and fill in the test tank, and thereafter, the supply of the gas was stopped. Then, the pressure of the gas accumulated within the test tank was measured at predetermined time intervals when the gas flew out towards the downstream side of the fluid path, thereby determining information (H1) regarding changes in the resulting gas pressure over time. Subsequently, the resulting time-dependent information (H1) was compared with time-dependent information (H0) regarding the gas pressure which was obtained in the same manner as described above through the measurement conducted initially upon introduction of the flow controller into the manufacturing line, thereby determining the amount of deviation between them. Then, this amount of deviation was analyzed to correct data regarding flow control, such as a reference voltage value (the valve drive control information), which was then delivered to the actuator of the flow control valve mechanism for flow rate control.
An example of such a flow controller that includes the function of testing flow rates is suggested in the invention disclosed in Japanese Patent Application Laid-Open No. 2006-38832, listed below, which was previously filed by the same applicant.
In Japanese Patent Application Laid-Open No. 2006-38832, a flow controller was suggested which was provided with a test valve for opening or closing the fluid path, a test tank having a predetermined capacity, and pressure detection means for detecting the pressure of a fluid passing through the fluid path and delivering the resulting pressure detection signal. The flow controller also included test control means that uses the test valve, the test tank, and the pressure detection means to control the flow rate test operation.
The flow controller disclosed in Japanese Patent Application Laid-Open No. 2006-38832 performs the flow rate test operation for the flow controller as follows. Namely, control can be performed on the current flow rate through the fluid path without removing the flow controller from the pipeline coupled to the semiconductor manufacturing apparatus. More specifically, it is possible to test the amount of deviation obtained by a comparison between the current flow rate and the initial flow rate determined at the time of installation of the flow controller in the manufacturing line. Then, based on this test result, the control data serving as a reference for flow rate control can be corrected. However, to perform this testing operation, it was inevitable to stop the operation of the semiconductor manufacturing apparatus. In other words, this test operation could not be performed while the semiconductor manufacturing apparatus, for example, a CVD apparatus was being used for deposition on wafers. Furthermore, to implement the flow rate test operation, not only the operation of the semiconductor manufacturing apparatus was required to stop but also a separate discharge gas line was required, in addition to the fluid path through which the process gas was supplied, to discharge the process gas out of the fluid path for the test operation.